1. Field of the Invention
The invention relates to a method and apparatus for determining the frequency of short oscillation bursts of electrical signals.
Within the scope of the present invention a short oscillation burst of an electrical signal means a fragment limited in time of some 10 or less oscillations of a narrow-band AC signal. Such an oscillation bunch is referred to as burst.
2. Description of the Prior Art
Whereas the determination of the frequency of a longer, i.e. theoretically infinitely long, narrow-band AC signal lasting practically more than several hundred oscillations generally presents no problems, the determination of the frequency of an oscillation burst consisting only of a few oscillations proves to be complicated, particularly for higher frequencies.
Thus, a period duration measurement, i.e. measurement of the time between two similar signal points, generally the zero passages of the signal, is only useful when the time basis available is of substantially higher frequency than the signal burst frequency. To form an average value, as is necessary with slight phase jitter of the signal, an external circuit must be employed.
For frequency counting, i.e. for counting out the oscillations in a time window, a highspeed and thus complicated counter circuit is necessary. With additional expenditure, such a counter circuit can be provided with a means for (analog) measurement of the phase error between the time window and the counting threshold.
In addition, methods are known such as the Fourier transformation or correlation analysis which are both very informative and also noise tolerant, but they require a relatively high expenditure.
Furthermore, a frequency determination can be made by a phase-synchronous lock loop (PLL) which however requires a high oscillation burst recurrence frequency and the individual oscillation burst frequencies must not lie too far apart. However, this makes it possible to a limited extent to obtain an average value over several oscillation bursts without additional expenditure.
Such methods and apparatuses are used in countless areas of technology for frequency determination of oscillation or signal bursts. An important area of use is laser Doppler velocimetry (LDV). As a rule, for the signal evaluation here a personal computer is employed which performs parts of the evaluations described but is also intended to form a mean value and various other statistical quantities of successive signal burst frequencies.